1. Field of the Invention
The present invention relates to an optical DQPSK receiver apparatus for receiving an optical signal modulated by DQPSK. The present invention is applicable to optical communications, optical signal processing, and optical measurements.
2. Description of the Related Art
Phase modulation has been in practical use as one of technologies to transmit signals in optical transmission systems. In the phase modulation, data is transmitted by shifting a phase of carrier wave in accordance with transmission data. For example, in Quadrature Phase Shift Keying (QPSK), respective “θ”, “θ+π/2”, “θ+π”, or “θ+3π/2” is assigned to each of the corresponding symbol “00”, “01”, “11” or “10” comprised by 2-bit data. Here, “θ” is an arbitrary phase. The receiver equipment recovers transmitted data by detecting the phase of the received signal.
Differential Quadrature Phase Shift Keying (DQPSK) is known as a technology to realize a QPSK receiver in a relatively easy way. In DQPSK, the amount of change in the phase of a carrier wave between the value of a symbol transmitted previously and the value of a symbol transmitted next (“0”, “π/2”, “π”, or “3π/2”) is associated with the 2-bit transmission information. Therefore, the receiver equipment can recover transmission data by detecting the phase difference between the adjacent two symbols.
FIG. 1 is a diagram describing the configuration of an existing optical DQPSK receiver apparatus. An optical DQPSK receiver apparatus 1000 shown in FIG. 1 comprises a pair of interferometers 1001 and 1002, and an optical DQPSK signal is split by an optical splitter and guided to a pair of the interferometers 1001 and 1002. A 1-symbol delay element is provided to one arm of the interferometer 1001, and a π/4 phase shift element is provided to the other arm. A 1-symbol delay element is provided to one arm of the interferometer 1002, and −π/4 phase shift element is provided to the other arm. A photo detector circuit 1003 comprises a pair of photodiodes 1003a and 1003b connected in series with each other. A photo detector circuit 1004 comprises a pair of photodiodes 1004a and 1004b connected in series with each other. A pair of optical signals output from the interferometer 1001 is guided to the photodiodes 1003a and 1003b, and a pair of optical signals output from the interferometer 1002 is guided to the photodiodes 1004a and 1004b. Note that to connect in series refers to the state of two diodes being in series when the two diodes are modeled as current sources.
The photo detector circuits 1003 and 1004 output a difference in current generated by each of a pair of photodiodes (differential signal.). The differential signals generated by the photo detector circuits 1003 and 1004 are amplified by each of preamplifiers 1005 and 1006, and are transmitted to a signal process circuit. The signal process circuit recovers 2-bit data from these differential signals.
FIG. 2 is a diagram explaining operations of the optical DQPSK receiver apparatus shown in FIG. 1. In DQPSK, the phase difference between adjacent symbols is “0”, “π/2”, “π”, or “3π/2”. If the phase differences in the optical signals interfering with each other in the interferometers 1001 and 1002 are “Δφa” and “Δφb”, respectively, the current generated by each photodiode is as shown in FIG. 2. In this case, the current value is normalized. Each of the photo detector circuits 1003 and 1004, then, output signal representing the difference between the current generated by a pair of photodiodes. Therefore, the output levels of the photo detector circuits 1003 and 1004 becomes “0.7” or “−0.7” in accordance with the phase difference between symbols adjacent to each other. Therefore, by using “threshold=0”, 2-bit data corresponding to the phase difference between the symbols can be obtained. In the example of FIG. 2, for example, when the phase difference between symbols is “π/2”, the output levels of the photo detector circuits 1003 and 1004 are “−0.7(<0)” and “0.7(>0)”, and therefore, “0” and “1” are obtained.
The configuration and the operation of the optical DQPSK receiver apparatus are described in detail in a patent document 1 (published Japanese Translation of PCT patent application No. 2004-516743 (WO2002/051041 or US2004/0081470)) and a patent document 2 (WO 03/063515A2), for example.
In the conventional optical DQPSK receiver apparatus shown in FIG. 1, the amount of phase shift of a pair of interferometers 1001 and 1002 is “π/4” and “−π/4”, and the difference between them is “π/2”. However, when the difference in the amount of phase shift deviates from “π/2”, the reception quality is degraded.
FIG. 3 is a diagram showing a relation between the difference in the amount of phase shift of a pair of interferometers and the recovered signal. In FIG. 3, the horizontal axis represents the deviation of the difference in the amount of phase shift from “π/2”. The vertical axis represents the output level of the photo detector circuits 1003 and 1004. Here, if the difference in the amount of phase shift is “π/2 (horizontal axis=0)”, the output level of the photo detector circuit is “0.7” or “−0.7”. Therefore, using “threshold=0”, the logical value of the output signal of the photo detector circuit (0 or 1) can be determined. However, as the deviation from “π/2” of the difference in the amount of phase shift becomes large, the output level of the photo detector circuit 1003 or 1004 is closer to zero. In other words, the difference between the output level of the photo detector circuit and the threshold becomes small. As a result, the probability of the error in decision of the logical value of the output signal of the photo detector circuit becomes high, and the reception quality is degraded. When the difference in the amount of a pair of the interferometers becomes “π/4 (or 3π/4)”, the output level of one photo detector circuit becomes “0”. In such a case, the logical value (0 or 1) of the output signal of the photo detector circuit cannot be determined using “threshold=0”, and therefore, data cannot be recovered.
As explained above, in the conventional optical DQPSK receiver apparatus, when the amount of phase shift of the phase shift element in an interferometer deviates, reception quality is degraded. Note that the amount of phase shift of the phase shift element changes due to the thermal change and aging degradation etc.
When the optical DQPSK receiver apparatus receives high-speed data of several ten Gbps, the frequency characteristics of the photodiode constituting the photo detector circuits 1003 and 1004 have to be superior. However, such a high-speed photodiode is highly expensive in general. In other words, if the number of photodiode constituting the photo detector circuit can be reduced, it is possible to keep the cost of the optical DQPSK receiver apparatus low.